Stop building semantic highlights on the main thread (#57264)

Trace on `main` that showed the hogged main thread: 
[Sample of Zed
Nightly.txt](https://github.com/user-attachments/files/28060310/Sample.of.Zed.Nightly.txt)


Release Notes:

- N/A
80c0f7de0aKirill Bulatov committed on 5/20/2026, 2:12:42 PM· committed by GitHubparentdd528e3
1 file changedLine totals unavailable